1. Field of the Invention
The present invention generally relates to a braced steel frame that is utilized in a structure that is subject to seismic loads. In particular, the braced steel frame is a pin-fused frame that lengthens dynamic periods and reduces the forces that must be resisted within the frame so that the frame can withstand seismic activity without sustaining significant damage.
2. Description of the Related Art
Structures have been constructed, and are being constructed daily, in areas subject to extreme seismic activity. Special considerations must be given to the design of such structures. In addition to normal loading conditions, the walls and frames of these structures must be designed not only to accommodate normal loading conditions, but also those loading conditions that are unique to seismic activity. For example, frames are typically subject to lateral cyclic motions during seismic events. To withstand such loading conditions, structures subject to seismic activity must behave with ductility to allow for the dissipation of energy under those extreme loads.
Conventional frames subject to seismic loads typically have been designed with the beams and braces fully connected to columns either by welding or bolting or a combination of the two. Flanges of beams are typically connected to column flanges via full penetration welds. Beam webs may be either connected with full penetration welds or by bolting. Diagonal bracing members are typically connected to a joint that is welded to the beams and the columns. Diagonal braces are typically bolted to the joints; however, welding is also used.
Braced frames have been used extensively in structures that resist lateral loads due seismic events. In addition, the use of moment-resisting frames in taller structures may not be feasible since the required stiffness may only be achievable with large structural members that add to the amount of material required for the structure and therefore cost. These frames provide an efficient means of achieving the appropriate stiffness, however provide questionable ductility when subjected to cyclic loadings. Since structural members are typically subjected to primarily axial loads with minimal bending, the material required to resist forces is usually low.
These conventional frames may be designed to have bracing members that resist only tension or that resist both tension and compression. Since ductility is limited in these frames, building codes, such as the Uniform Building Code (UBC), have limitations to their use. Tension-only braced frames (diagonal members only capable of resisting tensile loads) for occupied structures are limited by code to a height of 65 feet. In recognition of limited system ductility in this design, the recommended R-Factor for this system is 2.8 compared to 8.5 in a special moment-resisting frame (the higher the R-Factor the higher the potential system ductility in a seismic event).
Further, conventional braced frames that resist both tension and compression provide questionable ductility when subjected to cyclic seismic loading. The braces in these frames typically buckle and in some cases fracture when further subjected to tension and compression loads. For instance, in accordance with building codes, specifically the Uniform Building Code (UBC), braced frames capable of resisting both tension and compression are limited to a height of 160 feet for ordinary braced frames and 240 feet for special concentrically braced frames. In recognition of limited system ductility in design, the recommended R-Factor for ordinary braced frames is 5.6 and for special concentrically braced frames is 6.4, compared to 8.5 in a special moment-resisting frame. Eccentrically braced frames are designed to have the horizontal “linking” member inelastically deform during an extreme seismic event. This ductility for this frame is recognized by the UBC by recommending an R-Factor=7.0. The permanent deformation of the links within these frames raises serious questions about the structure's capability of resisting further seismic events without repair or replacement.
Recent testing of braced frames, particularly steel concentric braced frames (CBF), indicates that many commonly used members and brace configurations do not meet seismic performance expectations. Net member section properties, section type, width-thickness ratio of the member cross section, and member slenderness affect the ductility of the braces. This was shown through the research of Mahin and Uriz and documented in the “Seismic Performance Assessment of Concentrically Braced Steel Frames”, Proceedings of the 13th World Conference of Earthquake Engineering, 2004.
Considerable research has been performed considering the performance of braced frames, and developments of braced systems have been made that allow for inelasticity to occur in a prescribed location. Such systems include Buckling Restraint Braced Frames (BRBF), where devices are inserted in the braces allowing for inelasticity to occur in localized areas, typically at the ends of the brace. After a severe seismic event, these devices protect the diagonal member from uncontrolled buckling, but the braces must be removed and replaced to provide for future integrity of the structure. These braces are manufactured and supplied by Nippon Steel Corporation, Core-Brace Systems, and others.
Frames without diagonal braces provide additional ductility but with far less stiffness. Moment-resisting frame systems prove effective in resisting lateral loads when the frames are designed for the appropriate loads and the connections are detailed properly. In recent seismic events, including the Northridge Earthquake in Northridge, Calif., moment-resisting frames within structures that used welded flange connections successfully prevented buildings from collapsing but these frames sustained significant damage. After being subject to seismic loads, most of these types of moment-resisting frames have exhibited local failures of connections due to poor joint ductility. Such frames with such non-ductile joints have raised significant concerns about the structural integrity and the economic performance of currently employed moment-resisting frames after being subject to an earthquake.
Since the Northridge Earthquake, extensive research of beam-to-column moment connections has been performed to improve the ductility of the joints subject to seismic loading conditions. This research has lead to the development of several modified joint connections, one of which is the reduced beam section connection (“RBS”) or “Dogbone.” Another is a slotted web connection (“SSDA”) developed by Seismic Structural Design Associates, Inc. While these modified joints have been successful in increasing the ductility of the structure, these modified joints must still behave inelastically to withstand extreme seismic loading. It is this inelasticity, however, that causes joint failure and in many cases causes the joint to sustain significant damage. Although the amount of dissipated energy is increased by increasing the ductility, because the joints still perform inelastically, these conventional joints still tend to become plastic or yield when subject to extreme seismic loading.
Although current frames may resist seismic events and prevent collapse, the damage caused by the members and joints inability to function elastically, raises questions about whether structures that use these conventional designs can remain in service after enduring seismic events. A need therefore exists for frames that can withstand a seismic event without experiencing significant inelasticity or failure so that the integrity of the structure remains relatively undisturbed even after being subject to seismic activity.